In recent years, there has been increasing interest in magnetic random access memories (hereinafter also referred to as MRAMs) using the magnetoresistive effect of ferromagnetic materials as the next-generation solid-state nonmagnetic memories that are capable of high-speed reading and writing, and performing large-capacity operations with smaller power consumptions. Particularly, magnetoresistive elements having magnetic tunnel junctions have been attracting increasing attention, since the high magnetoresistance ratios of such elements were discovered.
The fundamental structure of a magnetic tunnel junction is a three-layer stacked structure formed by stacking a storage layer having a changeable magnetization direction, an insulating layer, and a fixed layer that is located on the opposite side from the storage layer and maintains a predetermined magnetization direction. When a current flows into such a magnetic tunnel junction, the flowing current tunnels through the insulating layer. At this point, the resistance of the junction varies in accordance with the relative angle between the magnetization directions of the storage layer and the fixed layer. The resistance of the junction becomes smallest when the magnetization directions are parallel to each other, and becomes largest when the magnetization directions are antiparallel to each other. The variation in resistance is called a tunneling magnetoresistance effect (hereinafter also referred to as the TMR effect). Where a magnetoresistance element having a magnetic tunnel junction is actually used as a memory cell, the parallel state and the antiparallel state of the magnetization directions of the storage layer and the fixed layer (or the minimum resistance and the maximum resistance) are associated with binary information “0” or “1”, to store information.
To perform writing on the memory of a magnetoresistive element, a known magnetic-field write method is used. By this method, a write wire is placed in the vicinity of a memory cell, and only the magnetization direction of the storage layer is reversed by the current magnetic field generated when a current is applied to the write wire. However, if the device size is made smaller so as to realize a large-capacity memory, the coercive force Hc of the magnetic material forming the storage layer becomes larger in principle. Therefore, the current required for writing tends to become larger as the device is made smaller. On the other hand, the current magnetic field generated from the write wire becomes smaller in principle with a reduction in cell size. Therefore, by the magnetic-field write method, it is difficult to achieve both a reduction in cell size and a reduction in write current as required in a large-capacity design.
Meanwhile, as a write method to overcome the above problem, a write method (a spin-injection write method) using a spin momentum transfer (SMT) has been recently suggested. By this method, a spin-polarized current is applied to a magnetoresistive element, so as to reverse the magnetization direction of the storage layer. As the volume of the magnetic layer forming the storage layer becomes smaller, the amount of spin-polarized electrons can be made smaller. Therefore, this method is expected to be the write method for realizing a smaller device size and a lower current at the same time.
However, if the device is made smaller to achieve a larger capacity, the energy barrier or the magnetic anisotropy energy for maintaining the magnetization direction of the storage layer in one direction becomes smaller than the thermal energy. As a result, the magnetization direction of the magnetic material fluctuates (thermal disturbance), and the stored information can no longer be maintained.
In general, the energy barrier required for reversing a magnetization direction is expressed by the product of the magnetic anisotropy constant (the magnetic anisotropy energy per unit volume) and activation volume. Therefore, to secure a sufficient resistance to thermal disturbance in a minute device size area, it is necessary to select a material having a large magnetic anisotropy constant. The structures of in-plane magnetization types that have been studied so far normally use shape magnetic anisotropies. In such cases, to increase the magnetic anisotropy energy, methods need to be taken, such as increasing the aspect ratio of the magnetoresistive element, increasing the film thickness of the storage layer, and increasing the saturation magnetization of the storage layer. However, with the characteristics of spin-injection methods being taken into consideration, any of those methods leads to an increase in switching current, and is not suitable for miniaturization.
A material having a large magnetic crystalline anisotropy, instead of a shape magnetic anisotropy may be used. In that case, the easy axis of magnetization in the in-plane direction is distributed in the film plane. Therefore, the MR ratio (magnetoresistance ratio) becomes lower, or incoherent precessional motion is induced. As a result, the switching current increases. Therefore, this method is not preferable either. In a structure of an in-plane magnetization type, a magnetic anisotropy depending on the shape is used, and the switching current is sensitive to the shape. As a result, there is a possibility that the variation in the switching current increases as the structure becomes smaller.
On the other hand, a so-called perpendicular magnetization film having an easy axis of magnetization in a direction perpendicular to the film plane may be used as the ferromagnetic material forming a magnetoresistive element. When a magnetic crystalline anisotropy is used in a structure of a perpendicular magnetization type, shape anisotropies are not used. Therefore, the device shape can be made smaller than that of a device of an in-plane magnetization type. Further, the variation of the easy magnetization direction can be made smaller. Accordingly, by using a material having a large magnetic crystalline anisotropy, a smaller size and a lower current can be realized at the same time while a resistance to thermal disturbance is maintained.
Examples of materials to be used as perpendicular magnetization films include L10-ordered alloys (such as FePt and CoPt), artificial alloys (such as Co/Pt and Pd), hcp materials (such as CoPt), and RE-TM materials (such as Tb—CoFe).
As known generally, the switching current for reversing magnetization by a spin-injection method normally depends on the saturation magnetization Ms and magnetic damping constant α of the storage layer. Therefore, to reverse the magnetization of the storage layer by a spin injection with a low current, it is critical to reduce the saturation magnetization Ms and the magnetic relaxation constant α. Further, a magnetoresistive element needs to endure the processing temperature as a device. However, none of the above described perpendicular magnetization films satisfy all the above characteristics required in the storage layer.